Oxidation resistant separator for a battery

ABSTRACT

A method for preventing oxidation of a polyolefin separator in a lithium ion secondary battery includes the steps of: providing a lithium ion secondary battery having a positive electrode and a polyolefin separator film; and positioning an antioxidative barrier coating between the positive electrode and the polyolefin separator film, the antioxidative barrier coating being made of a polymer having a resistance to oxidation greater than polyethylene.

RELATED APPLICATIONS

This application is a division of co-pending application Ser. No.11/549,273 filed Oct. 13, 2006, now U.S. Pat. No. ______, which is adivision of U.S. application Ser. No. 10/371,461 filed Feb. 21, 2003,now abandoned.

FIELD OF THE INVENTION

The invention is directed to reducing or preventing oxidation of themicroporous membrane separator used in a rechargeable lithium-ionbattery.

BACKGROUND OF THE INVENTION

Rechargeable (or secondary) lithium ion batteries (hereinafter lithiumion batteries) are commonly used today in, for example, hand held(cellular) telephones and laptop computers, among other things. Thoselithium batteries are favored because of their high energy density, highvoltage, and good charge retention. These batteries typically use alithiated carbon material as the negative electrode, intercalationcompounds, such as transition metal oxides (e.g., Li_(x)CoO₂), as thepositive electrodes, microporous polyolefin membranes as the separatorbetween the electrodes, and liquid electrolyte contained within thepores of the membrane and in electrochemical communication between theelectrodes (reference to positive and negative is during discharge).Further detail on the materials of construction of these variouscomponents may be found in Linden, D., Editor, Handbook of Batteries,2nd Edition, McGraw-Hill, Inc., New York, N.Y. 1995, pp. 36.1-36.77, andBesenhard, J. O., Editor, Handbook of Battery Materials, Wiley-VCHVerlag GmbH, Weinheim, Germany, 1999, for example pp. 47-55, each ofwhich is incorporated herein by reference. These batteries aredistinguished from so called “lithium polymer” batteries that arecharacterized by electrolytes that are in the form of a gel or solid,and consequently, have lower conductivities. Some such separators aredescribed in Besenhard, J. O., Ibid., pp. 557-558, incorporated hereinby reference.

When the lithium ion battery is fully charged, the positive electrode(cathode) becomes a strong oxidizing agent because of its high positivevalence, thereby creating at the positive electrode/separator interfacea very tough environment for the battery components (electrodes,electrolytes, and separators). All these components are susceptible todegradation, via oxidation, in this environment.

Oxidation of the separator is undesirable. The separator serves severalfunctions, one is to insulate the electrodes from one another, i.e.,prevent internal shorting. This insulating function is accomplished bythe use of polyolefin membranes. When a polyolefin separator isoxidized, it looses its physical and chemical integrity and is thusunsuitable for its original intended function. This shortens the usefullife of the battery because the battery no longer can hold its chargedue to internal shorting within the battery.

This oxidative environment at the positive electrode/separator interfacemay be more fully understood with reference to the following.

For example, a typical lithium ion battery may have: a positiveelectrode (cathode) containing lithium cobalt oxide, lithium nickeloxide, or lithium manganese oxide (Li_(x)CoO₂ will be discussedhereinafter); a negative electrode (anode) containing a lithiatedcarbon; a liquid electrolyte containing a lithium salt (e.g., LiPF₆ orLiClO₄) in an aprotic organic solvent (mixtures of EC, DEC, DMC, EMC,etc); and a microporous polyolefin membrane. During discharge, lithiumions migrate from the negative electrode (anode) containing thelithiated carbon to the positive electrode (cathode) containingLi_(x)CoO₂. The cobalt is reduced from a +4 valence to a +3 valence, andcurrent is generated. During charging, current is supplied to thebattery at a voltage in excess of the discharge voltage to move thelithium that migrated to positive electrode (cathode) back to negativeelectrode (anode), and the cobalt is oxidized from the +3 valence to the+4 valence state. In commercial batteries, a fully charged batterytypically consists of about 75% of the cathodic active material (e.g.,cobalt) to be at the +4 valence state and, if Li_(x)CoO₂ is used, x isabout 3.5. In this state, the cobalt of the positive electrode (cathode)is a strong oxidizing agent. It can and will attack materials around it,particularly the separator.

Separator oxidation can be seen. FIG. 1 is a photograph of the magnifiedimage of an oxidized separator. The separator is a microporouspolyethylene membrane made by a ‘wet’ or ‘phase inversion’ process. Thisseparator was recovered from a fully charged cell that had been storedin an oven (85° C.) for three days. The dark areas are the oxidizedareas. FIG. 2 is a schematic illustration of the cross section of themembrane shown in FIG. 1. It is believed that these dark areas (whichmay also be described as: charred or partially charred (e.g., see FIG.2) or oxidized or partially oxidized or oxidized polyethylene orpartially oxidized polyethylene, e.g., a polyethylene material after atleast some oxidization) have less physical and chemical strength. Poormechanical strength can lead to shorts and thus battery failure.

The foregoing oxidation problem is common. When batteries are stored ina fully charged condition, when batteries are stored, at temperaturesgreater than room temperatures, in a fully charged condition, or whenbatteries are charged at a constant voltage ^(˜)4.2V for an extendedperiod of time, the oxidation problem arises. The latter situation iscommon, for example, when a laptop computer is left ‘plugged in’ andtherefore continuously charging. In the future, the oxidation problemmay become more severe. The current trend is for these batteries to beable to operate at temperatures greater than room temperature and forthese to be stored, fully charged, at temperatures greater than roomtemperature. Therefore, oxidation at these greater potentialtemperatures will likely be more severe.

Accordingly, there is a need for batteries and separators that resistoxidation at the positive electrode (cathode)/separator interface of alithium ion battery.

SUMMARY OF THE INVENTION

A method for preventing oxidation of a polyolefin separator in a lithiumion secondary battery includes the steps of: providing a lithium ionsecondary battery having a positive electrode and a polyolefin separatorfilm; and positioning an antioxidative barrier coating between thepositive electrode and the polyolefin separator film, the antioxidativebarrier coating being made of a polymer having a resistance to oxidationgreater than polyethylene.

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form that is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a photograph of an oxidized separator.

FIG. 2 is a schematic illustration of the cross section of the separatorshown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, the battery is a rechargeable lithium ion battery.Such batteries are well known as is demonstrated by reference to Linden,Handbook of Batteries, 2nd Edition, McGraw-Hill, Inc., New York, N.Y.,1995, and Besenhard, Handbook of Battery Materials, Wiley-VCH VerlagGmbH, Weinheim, Germany, 1999, both incorporated herein by reference.

The rechargeable lithium ion battery referred to herein may be anyrechargeable lithium ion battery. These batteries may be, for example,cylindrical, prismatic (rectangular), or pouch type gel polymer cells.Rechargeable lithium ion batteries with liquid electrolytes, however,are preferred. Liquid electrolytes are used to distinguish thesebatteries from lithium gel or polymer batteries that use gel or solidelectrolytes. The batteries with liquid electrolytes are commerciallyavailable and include, but are not limited to, types 14500, 16530,17500, 18650, 20500, 652248, 863448, 143448, and 40488.

The negative electrode adapted to give up electrons during discharge isany material conventionally used in a negative electrode in rechargeablelithium batteries. Such materials are lithium metal, lithium alloy,lithiated carbons, and transition metal compounds. For example, thelithium alloy may be LiAl. The lithiated carbons (intercalation ofcarbon) may be Li_(0.5)C₆ or LiC₆, where the carbon is, for example,coke or graphite. The transition metal compounds may be LiWO₂, LiMoO₂,LiTiS₂. The lithiated carbons are preferred.

The positive electrode adapted to gain electrons during discharge is anymaterial conventionally used in a positive electrode in rechargeablelithium batteries. Such materials are characterized as having: high freeenergy of reaction with lithium, wide ability for intercalation, littlestructural change on reaction, highly reversible reaction, rapiddiffusion of lithium in solid, good electronic conductivity, nosolubility in electrolyte, and readily available or easily synthesizedfrom low-cost materials. Such materials include, for example, MoS₂,MnO₂, TiS₂, NbSe₃, LiCoO₂, LiNiO₂, LiMn₂O₄, V₆O₁₃, V₂O₅. The preferredmaterials include LiCoO₂, LiNiO₂, LiMn₂O₄. The most preferred is LiCoO₂.

The microporous separator is sandwiched between the negative electrodeand the positive electrode. These separators are typically made frompolyolefins, but other film-forming polymer may be used. The polyolefinsinclude polyethylene (including LDPE, LLDPE, HDPE, and UHMWPE),polypropylene (PP), polymethyl pentene (PMP), polybutylene (PB),copolymer thereof, and mixtures of any of the foregoing. Theseseparators may be made by either a dry stretch (Celgard) process or awet (or phase inversion or extraction) process. Such separators arecommercially available from Celgard Inc. of Charlotte, N.C., TonenChemical Corporation of Tokyo, Japan, Asahi Kasei Corp. of Tokyo, Japan,and Ube Industries of Tokyo, Japan. Such separators may be singlelayered or multi-layered. Single layered HDPE and UHMWPE separator andPP/PE/PP multi-layered separators are preferred.

The electrolyte may be any conventionally known electrolyte. Suchelectrolyte may be characterized by good ion conductivity (>10⁻³ S/cmfrom −40 to 90° C.) to minimize internal resistance, a lithium iontransference number approaching unity, a wide electrochemical voltagewindow (0-5V), thermal stability, and compatibility with other cellcomponents. Preferably, the electrolyte is a liquid organic electrolyte.The electrolyte comprises a solvent and a salt. The solvents (also knownas aprotic solvents) may include, but are not limited to, butyrolacetone(BL), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate(PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethylcarbonate (DEC), diethoxyethane (DEE), ethyl methyl carbonate (EMC) andmixtures thereof. The salts may include, but are not limited to, LiPF₆,LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₃, LiBF₆, LiClO₄, and mixtures thereof. Thepreferred electrolytes include: LiPF₆ in EC/DEC; LiBF₄ in EC/DMC; andLiPF₆ in EC/EMC. The most preferred electrolytes include: LiPF₆ inEC/EMC and LiBF₄ in EC/EMC.

The present invention is directed to reducing or eliminating oxidationthat occurs at the interface between the positive electrode and theseparator. The reduction or elimination of oxidation that occurs at theinterface between the positive electrode and the separator improves thecell's performance. Performance enhancements occur in cycle lifecharacteristics, and shelf life characteristics at low and high voltagesand at low and high temperatures, but especially at high temperatures(e.g., >35° C., especially >55° C. For this purpose, an antioxidativebarrier is interposed between the separator and the positive electrodeto prevent oxidation of the separator. Several such barriers, forexample, are set out below.

The microporous separator may be made of a polymer that is resistant tooxidation, and that polymer must be in contact with the positiveelectrode. Here, the polymer of the separator is the antioxidativebarrier, and it is integral with the separator. Such polymers includepolypropylenes and halocarbons, e.g., polyvinylidene fluoride (PVDF),polytetrafluoro ethylene (PTFE), and copolymers of halocarbons. Suchpolymers must have a greater resistant to oxidation than polyethylene.

The separator may have a discrete polymer coating formed onto aseparator, and that polymer coating must be in contact with the positiveelectrode. Here, the discrete polymer coating is the antioxidativebarrier. Such polymer, as above, includes polypropylene, halocarbons,e.g., polyvinylidene fluoride (PVDF), polytetrafluoro ethylene (PTFE)and copolymers of halocarbons, and metal oxides, e.g., Al₂O₃ and TiO₂.In this solution, the coating is formed on any conventional separator,discussed above for example, by any conventional means. The coating maybe very thin, e.g., one molecule thick, and should not impede themovement of ions across or through the separator. A coating of PVDF, forexample, may be <0.4 mg/cm². Accordingly, the coating must besufficiently thick to inhibit oxidation of the separator, but not sothick as to unduly inhibit ion flow across the separator (i.e., undulyincrease internal resistance within the cell).

The positive electrode (cathode) may have a discrete polymer coatingformed thereon, and that polymer coating must be in contact with theseparator. Here, the discrete polymer coating is the antioxidativebarrier. Such polymer, as above, includes polypropylene, halocarbons,e.g., polyvinylidene fluoride (PVDF), polytetrafluoro ethylene (PTFE)and copolymers of halocarbons, and metal oxides, e.g., Al₂O₃ and TiO₂.In this solution, the coating is formed on any positive electrode,discussed above for example, by any conventional means. The coating maybe very thin, e.g., one molecule thick, and should not impede themovement of ions across or through the interface between the separatorand the positive electrode. A coating of PVDF, for example, may be <0.4mg/cm². Accordingly, the coating must be sufficiently thick to inhibitoxidation of the separator, but not so thick as to unduly inhibit ionflow (i.e., unduly increase internal resistance within the cell).

The microporous separator may include antioxidants in the polymericmaterial. These antioxidants may be dispersed throughout the polymericmaterial forming the separator, but preferably it should be concentratedat the face of the separator that will be juxtaposed to the positiveelectrode, to maximize the efficacy of the antioxidant. Antioxidants areroutinely added to polymers prior to processing. These antioxidantsprotect the polymer during the rigors of processing (e.g., extrusion,typically melt extrusion), as well as, subsequently during use, that isexposure to the atmosphere. Those antioxidants may be useful inprotecting the separator from the instant problem, but the initialconcentration of antioxidant added before processing should be increasedsignificantly. The significant increase (perhaps, greater than 100times) is needed because during processing 70-80% of the antioxidantconventionally added to the polymer is sacrificed to protect thepolymer. Thus an insufficient amount is left after processing toadequately protect the separator. Accordingly, if the conventionaladdition rate for a particular antioxidant was 0.01-0.1% by weight ofthe polymer, then with the instant invention, the addition rate may beincreased to 1-10% by weight. Of course, the antioxidant should bepresent in an amount sufficient to inhibit oxidation of the separator atthe interface between the positive electrode and the separator. Anadditional consideration is that the antioxidant should not be solublein the electrolyte. Such antioxidants include, but are not limited to,for example: phenols; phosphorous containing compounds (phosphates,phosphonites); and sulfur containing compounds (thiosynergists).Examples of such antioxidants include, but are not limited to, IRGANOX1010, IRGAFOS 168, IRGANOX B-125, and IRGANOX MD 1-24, each commerciallyavailable from Ciba-Geigy Corporation of Cranberry, N.J. The use ofantioxidants is preferred when the polymeric material is polyethylene(including LLDPE, LDPE, HDPE, and UHMWPE).

The separator may have a discrete antioxidant coating formed thereon,and that coating must be in contact with the positive electrode. Thecoating is, preferably, very thin, i.e., should not impede the movementof ions across or through the interface between the separator and thepositive electrode, and protects the mechanical integrity of theseparator by suppressing oxidative degradation. The coating may beapplied by any conventional coating method including, for example,brushing, spraying, via roller, or immersion. Of course, the antioxidantshould be present in an amount sufficient to inhibit oxidation of theseparator at the interface between the positive electrode and theseparator. Like above, these antioxidants should not be soluble in theelectrolyte. Such antioxidants include, but are not limited to, forexample: phenols; phosphorous containing compounds (phosphates,phosphonites); and sulfur containing compounds (thiosynergists).Examples of such antioxidants include, but are not limited to, IRGANOX1010, IRGAFOS 168, IRGANOX B-125, and IRGANOX MD 1-24, each commerciallyavailable from Ciba-Geigy Corporation of Cranberry, N.J. The use ofantioxidants is preferred when the polymeric material is polyethylene(including LLDPE, LDPE, HDPE, and UHMWPE).

The positive electrode may have a discrete antioxidant coating formedthereon, and that coating must be in contact with the separator. Thecoating is, preferably, very thin, i.e., should not impede the movementof ions across or through the interface between the separator and thepositive electrode, and protects the mechanical integrity of theseparator by suppressing oxidative degradation. The coating may beapplied by any conventional coating method including, for example,brushing, spraying, via roller, or immersion. Of course, the antioxidantshould be present in an amount sufficient to inhibit oxidation of theseparator at the interface between the positive electrode and theseparator. Like above, these antioxidants should not be soluble in theelectrolyte. Such antioxidants include, but are not limited to, forexample: phenols; phosphorous containing compounds (phosphates,phosphonites); and sulfur containing compounds (thiosynergists).Examples of such antioxidants include, but are not limited to, IRGANOX1010, IRGAFOS 168, IRGANOX B-125, and IRGANOX MD 1-24, each commerciallyavailable from Ciba-Geigy Corporation of Cranberry, N.J. The use ofantioxidants is preferred when the polymeric material is polyethylene(including LLDPE, LDPE, HDPE, and UHMWPE).

The present invention may be embodied in other forms without departingfrom the spirit and the essential attributes thereof, and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicated the scope of the invention.

We claim:
 1. A method for preventing oxidation on a polyolefin separatorin a lithium ion secondary battery comprising the steps of: providing alithium ion secondary battery having a positive electrode and apolyolefin separator film; and positioning an antioxidative barriercoating between the positive electrode and the polyolefin separatorfilm, the antioxidative barrier coating being made of a polymer having aresistance to oxidation greater than polyethylene.
 2. The method ofclaim 1 wherein the polymer is a halocarbon.
 3. The method of claim 1wherein the polymer comprises polyvinylidene fluoride.
 4. The method ofclaim 1 wherein the polymer consists of polyvinylidene fluoride.
 5. Themethod of claim 1 wherein the polymer is polypropylene.
 6. The method ofclaim 1 wherein the antioxidative barrier coating has a weight of <0.4mg/cm².
 7. The method of claim 1 wherein the antioxidative barriercoating has a weight of <0.15 mg/cm².
 8. The method of claim 1 whereinthe polyolefin separator film is made of polyethylene, copolymersthereof and mixtures thereof.
 9. The method of claim 8 wherein thepolyolefin separator film is a single layered film.
 10. The method ofclaim 8 wherein the polyolefin separator film is a multi-layered film.11. The method of claim 1 wherein the antioxidative barrier coating is adiscrete polymer coating formed onto the polyolefin separator filmand/or the positive electrode by a coating method selected from thegroup consisting of brushing, spaying, applying via roller, and applyingby immersion.
 12. The method of claim 1 wherein the antioxidativebarrier coating is formed on the polyolefin separator film.
 13. Themethod of claim 1 wherein the antioxidative barrier coating is formed onthe positive electrode.
 14. The method of claim 1 wherein the polymer isincluded in the polyolefin separator film.
 15. A method for preventingoxidation on a polyolefin separator in a lithium ion secondary batterycomprising the steps of: providing a lithium ion secondary batteryhaving a positive electrode and a polyolefin separator film; adding anantioxidative barrier between the positive electrode and the polyolefinseparator film, the antioxidative barrier including a polymer having aresistance to oxidation greater than polyethylene to the film: andpositioning the polymer toward the positive electrode.
 16. The method ofclaim 15 wherein the antioxidative barrier is a discrete polymer coatingformed on the polyolefin separator film.
 17. A method for preventingoxidation on a polyolefin separator in a lithium ion secondary batterycomprising the steps of: providing a lithium ion secondary batteryhaving a positive electrode and a polyolefin separator film; positioningan antioxidative barrier coating between the positive electrode and thepolyolefin separator film, the antioxidative barrier coating is adiscrete polymer coating formed on the polyolefin separator film and/oron the positive electrode, the antioxidative barrier coating includes apolymer having a resistance to oxidation greater than polyethylene.